1. Field of the Invention
The present invention relates generally to intrusion detection systems; and more particularly to such systems having a pair of sensors, each with two elements, and means responsive to a predetermined sequence of actuation of the elements to output an intruder-present signal.
2. Description of the Prior Art
Active intrusion detection systems typically include a radiation transmitting device and a separate sensor coupled to follow-on electronics for detecting disturbance of the received radiation. On the other hand, passive intrusion detection systems function by sensing a threshold change in the amount of radiation present whenever there is an intrusion into the protected area. However, such systems are susceptible to false intruder-present signal due to changes in ambient light, temperature changes, drafts, etc.
In an attempt to overcome the cause of such false signals, many such systems include a so called "dual element" sensor; which is a single sensor having a pair of opposite polarity elements which view closely adjacent portions of the protected area to produce polarized outputs characteristic of the change in infrared content of their respective fields of view. The dual elements are aligned sequentially in the direction of expected intrusion. When both elements are affected at the same time, their outputs are negated by mutual subtraction of the element outputs. Accordingly, the occurrence of false intruder-present signals resulting from changes in the ambient conditions is reduced. On the other hand, the logic produces an intruder-present signal in response to a sequence of opposite polarity pulses such as would be caused by an intruder walking across the field of view of the two elements.
Reliability of intrusion detection systems, and security from false intruder-present signals, has been enhanced with the advent of detection systems comprising two separate channels "A" and "B", each channel having an associated sensor with a pair of sensor elements as described above. By requiring that intruder-present signal outputs from both sensors are needed in order for the system to respond, the risk of false triggering from electrical noise or non-intruder related changes in infrared radiation sources is reduced because there is less likelihood of spurious signals occurring in both channels.
For example, U.S. Pat. No. 4,704,533, which issued to Rose et al. on Nov. 3, 1987, discloses a two channel (A and B), dual sensor intrusion detection system arranged such that interdigited polarized elements of the sensors create adjacent detection zones and produce outputs referred to as +A, -A, +B, and -B. An intruder recognition circuit responds to sequential activation of one element from each sensor. That is, the system will respond to a pattern comprised of +A alone, followed by +A together with +B, followed by +B alone. The system will also respond to a pattern comprised of -A alone, followed by -A together with -B, followed by -B alone. While this system of responding to particular patterns of output signals has certain benefits in detecting intruders, it requires that the elements of the two sensors be closely adjacent. Also, by requiring output signals from only two elements of the four element system to actuate an intruder-present signal, the risk of false signals is greater than if more than two elements had to be activated in proper sequence for an intruder-present signal to be produced.